If you want to add powernv feature just because you need to distinguish
if the host supports KVM or not, there's much better way... In guest
section of capabilities XML, KVM support is indicated by <domain
type='kvm'> element. And that's what existing apps already use to detect
KVM presence/absence.

As my understanding, there is still some difference from 'kvm' capability.
'powernv' is only considered as one CPU feature of PPC64.
If other PPC platforms support KVM in the future, this feature can be
used to identify whether migration can be executed . :)

Well, as I already said, unless there is a way to explicitly
enable/disable powernv feature in guest CPU, I don't see any reason for
exposing the feature to users/apps. We definitely don't want apps/users
to detect support for migration (or anything else) by checking host CPU
features. If migration is not supported, then any migration API can fail
with VIR_ERR_OPERATION_UNSUPPORTED. That said, powernv feature can be
used internally by libvirt to check if some configurations/operations
are supported but it doesn't have to be exposed to users/apps. The CPU
driver stuff is there for configuring *guest* CPU.

I see, got it. This feature can be removed.
So it is not necessary to add <feature> in CPU driver for ppc64.
Thanks.